Image forming apparatus with voltage polarity switching means

ABSTRACT

An image forming apparatus includes a first image bearing member on which a toner image can be formed; an intermediate transfer member onto which the toner image is transferable from the first image bearing member; wherein the toner image is further transferred onto a second image bearing member therefrom; wherein the intermediate transfer member comprises a conductive layer to which a voltage is applied, a surface layer on the conductive layer, and a volume resistivity of the surface layer is 10 5  -10 11  ohm.cm.

This application is a continuation of application Ser. No. 08/407,869,filed Mar. 21, 1995, now abandoned, which is a divisional of applicationSer. No. 08/215,666, filed Mar. 22, 1994, now U.S. Pat. No. 5,438,398,issued Aug, 1. 1995, which is a continuation of application Ser. No.08/069,379, filed Jun. 1, 1993, now abandoned.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to an image transfer type image formingapparatus such as a copying machine, printer or facsimile machine inwhich a transferable (visualized) image of image information is formedon a first image bearing member through an image forming process, andthe transferable image is transferred onto a second image bearingmember, and is outputted as a print, or wherein a transferable imageformed on the first image bearing member is once transferred onto anintermediate transfer member, and is further transferred onto a secondimage bearing member, and is outputted as a print.

Here, the first image bearing member is either an electrophotographicphotosensitive member, an electrostatic recording dielectric member, amagnetic recording magnetic member or the like. The image formingprocess is an electrophotographic process, an electrostatic recordingprocess, magnetic recording process or the like.

The second image bearing member is either a transfer material, arecording sheet, a print sheet or another sheet material.

Referring first to FIG. 21, there is shown an example of an imageforming apparatus using an intermediate transfer member. The imageforming apparatus of this example is a color image forming apparatus(copying machine or laser beam printer) using an electrophotographicprocess.

Designated by a reference numeral 1 is an electrophotographicphotosensitive member (photosensitive drum) as a repetitively usableimage bearing member in the form of a rotatable drum, and is rotated ina direction indicated by an arrow (clockwise direction) at apredetermined peripheral speed (process speed).

During the rotation, the photosensitive drum 1 is uniformly charged to apredetermined potential and predetermined polarity by a primary charger(corona discharger) 2, and is then exposed to image light by an imageexposure means (not shown), such as a color separation and imagingexposure optical system for a color original, or a scanning exposuresystem having a laser scanner for producing a laser beam modulated inaccordance with a time series electric digital picture element signalcorresponding to image information. By doing so, a first color separatedimage of the intended color image (magenta component image) is formed asan electrostatic latent image.

The electrostatic latent image is developed with a first color, that is,magenta toner M (coloring charged particles) by a first developingdevice 41 (magenta developing device). At this time, the second tofourth developing devices 42, 43 and 44 (cyan, yellow and blackdeveloping devices) are not operated so that they do not act on thephotosensitive drum, and therefore, the first color magenta toner imageis not influenced by the second to fourth developing devices 42-44.

Designated by a reference numeral 5 is an endless intermediate transferbelt (intermediate transfer member), which is stretched around aconductive roller 6, two turn rollers 71 and 72, namely, around threerollers 6, 71 and 72. The conductive roller 6 keeps the belt 5 inpress-contact with the photosensitive drum 1 with a predeterminedpressure. Between the photosensitive drum 1 and the intermediatetransfer belt 5, a transfer nip N is formed (transfer position).

The intermediate transfer belt 5 is rotated in the indicated clockwisedirection at the same peripheral speed as the photosensitive drum 1. Tothe conductive roller 6, a transfer bias of the polarity (positive)opposite from the polarity of the charged toner (negative) on thephotosensitive drum 1, from a first bias voltage source 61. Theintermediate transfer belt may be of a dielectric material film such aspolyester, polyethylene or the like material film, or a multi-layerdielectric film lined with a conductive material at the backsidethereof. While the first color (magenta) toner image on thephotosensitive drum 1 surface passes through the transfer nip N, it issequentially transferred onto a rotating intermediate transfer belt 5 byan electric field formed in the transfer nip by the transfer biasapplied to the conductive roller 6.

The surface of the photosensitive drum 1, after the first color magentatoner image is transferred onto the intermediate transfer belt 5, iscleaned by a cleaning device 14.

Similarly, the following steps are carried out:

charging of the rotatable photosensitive drum; image exposure 3 of thesecond component image (cyan component image, for example); developmentwith the cyan toner C of the second developing device 42 (cyandeveloping device); transfer of the cyan toner image (second colorimage) onto the intermediate transfer belt 5; and cleaning of thephotosensitive drum 1 surface by the cleaning device 14:

charging of the rotatable photosensitive drum 1; image exposure of thethird component image (yellow component image, for example); developmentwith yellow toner Y of the third developing device 43 (yellow developingdevice); transfer of the third color (yellow) toner image onto theintermediate transfer belt; and cleaning of the photosensitive drum 1surface by the cleaning device 14: and

charging of the rotatable photosensitive drum 1; image exposure of thefourth color component image (black component image, for example);development with black toner B of the fourth developing device 44 (blackdeveloping device); transfer of the fourth color (black toner) imageonto the intermediate transfer belt 5; and cleaning of the surface ofthe photosensitive drum 1 by the cleaning device 14.

The foregoing image forming-transfer cycle are sequentially carried out,so that the four toner images (magenta, cyan, yellow and black tonerimages) are sequentially and superposedly transferred onto the outersurface of the rotatable intermediate transfer belt 5, by which acombined color toner image (mirror image) is formed corresponding to theoriginal color image.

Subsequently, a transfer material (sheet) 24 as the second image bearingmember is fed one-by-one from a sheet cassette 9 by a sheet feedingroller 10, and is supplied to the transfer nip N at a predeterminedtiming by a pair of registration rollers along a transfer guide 12.

The bias voltage supplying voltage to the conductive roller 6 isswitched from the first voltage source 61 to the second bias voltagesource 62, so that the conductive roller 6 is supplied with a biasvoltage of the polarity which is the same as that of the combined colortoner image on the intermediate transfer belt 5, by which the combinedcolor toner image is sequentially transferred from the intermediatetransfer belt 5 onto the surface of the transfer material 24 supplied tothe transfer nip N. The transfer material 24 having been subjected tothe toner image transfer through the transfer nip N is introduced alonga conveyance guide 13 into a fixing device 15 where it is heated andpressed by a temperature-controlled fixing roller 16 and the pressingroller 17, so that the image is fixed thereon. Then, it is discharged asa color print.

Designated by a reference numeral 8 is a cleaning device for theintermediate transfer belt, and is normally in an inoperative positionrelative to the belt. However, after the toner image transfer to thetransfer material 24 is completed, it acts on the outer surface of thebelt 5 to clean it.

(1) The apparatus using a dielectric material belt 6 as an intermediatetransfer member as in FIG. 21, involves the following problems.

Disturbance of the image upon transfer:

In order to properly transfer the toner image from the photosensitivedrum 1 onto the intermediate transfer belt 5 of the electricallyinsulative, it is required to increase the transfer bias voltage appliedto the conductive roller 6, and therefore, a strong electric field isproduced in the transfer nip N and the space before and after the nip.

Therefore, the toner image on the photosensitive drum 1 is attractedonto the intermediate transfer belt 5 before the transfer nip N(upstream of the transfer nip N with respect to the transfer materialconveying direction), with the result that the toner scatters around theimage.

In addition, static electricity is produced due to the triboelectricitybetween the insulative intermediate transfer belt 5 and thephotosensitive drum, 1 or between the insulative intermediate transferbelt 5 and the transfer material 24, with the result that the separationdischarge mark appears on the intermediate transfer belt 5, thusdisturbing the image.

Furthermore if the voltage applied to the conductive roller 6 from thefirst or second bias source 61 or 62 to provide a strong electric field,the electric current leaks through a small defect such as a pin holewhich may exist in the intermediate transfer belt 5, with the result ofdamage to the intermediate transfer belt 5 or the photosensitive drum 1.

If such current leakage occurs, the toner image transfer is no longerpossible to the intermediate transfer belt 5 with the result being alocal defect on the toner image transferred onto the intermediatetransfer belt 5.

(2) When the toner image is transferred from the photosensitive drum 1to the intermediate transfer belt 5, the central drop-out of transfereasily occurs in the toner image T on the belt 5, as indicated by a inFIG. 23. This is because the surface of the photosensitive drum 1 has anattraction force to the toner, and this occurs more if the tonerattraction force to the belt 5 is smaller.

The same applies to the toner image transfer from the intermediatetransfer belt 5 to the transfer material 24 after the sequentialsuperposing transfer process for the first to fourth colors. Conversely,however, the central droplet occurs less if the toner deposition forceto the belt 5 is smaller. Therefore, in the image forming apparatususing such an intermediate transfer member 5, there has not been aproper intermediate transfer member providing proper toner depositionforce, and therefore it has been difficult to form an image on thetransfer material 24 without the central drop-out.

(3) In a color image forming apparatus using an intermediate transfermaterial, when a monochromatic image is produced, the toner image istemporarily transferred from the photosensitive drum to the intermediatetransfer member, and thereafter, it is transferred further onto thetransfer material, and therefore, the image forming speed is much lowerthan when the toner image is directly transferred from thephotosensitive drum onto the transfer material.

(4) In an image forming apparatus using an intermediate transfermaterial, the resistance of the intermediate transfer material changesdue to the variation of the ambient condition such as the temperature orhumidity, and therefore, the transfer property changes due to theambient condition change.

SUMMARY OF THE INVENTION

Accordingly, it is a principal object of the present invention toprovide an image forming apparatus which is capable of forming imageswithout toner scattering and image disturbance attributable to theseparation discharge.

It is another object of the present invention to provide an imageforming apparatus capable of forming images without central drop-out.

It is a further object of the present invention to provide an imageforming apparatus having a high image forming speed.

It is yet a further object of the present invention to provide an imageforming apparatus capable of forming proper images irrespective ofambient condition.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an image forming apparatus according to afirst embodiment of the present invention.

FIG. 2 is a sectional view of an image forming apparatus according to asecond embodiment of the present invention.

FIG. 3 schematically illustrates an image transfer process from anintermediate transfer roller onto a transfer material for formation ofcombined color toner images.

FIG. 4 schematically illustrates an image transfer process in the caseof a monochromatic print.

FIG. 5 is a sectional view of an image forming apparatus according to athird embodiment of the present invention.

FIG. 6 is a sectional view of an image forming apparatus according to afourth embodiment of the present invention.

FIG. 7 schematically shows an image transfer process in the case ofduplicate print.

FIG. 8 is a sectional view of an image forming apparatus according to afifth embodiment of the present invention.

FIG. 9 shows a relation between a peripheral speed of an intermediatetransfer roller and a degree of an image transfer drop-out.

FIG. 10 is a sectional view of an image forming apparatus according to aseventh embodiment of the present invention.

FIG. 11 is a sectional view of an image forming apparatus according toan eighth embodiment of the present invention.

FIG. 12 is a sectional view of an image forming apparatus according to aninth embodiment of the present invention.

FIG. 13 is a sectional view of an image forming apparatus according toan eleventh embodiment of the present invention.

FIG. 14 is a sectional view of a major part of an image formingapparatus according to a twelfth embodiment of the present invention.

FIG. 15 illustrates results of control of a transfer bias voltage to atransfer material in the image forming apparatus according to the 13thembodiment.

FIG. 16 shows a control sequence.

FIG. 17 is a sectional view of an major part of an image formingapparatus according to a 14th embodiment of the present invention.

FIG. 18 is a sectional view of an image forming apparatus according to a12th embodiment.

FIG. 19 illustrates an image transfer process from an intermediatetransfer roller to a transfer material for formation of a combined colortoner image.

FIG. 20 schematically illustrates an image transfer process in the caseof a monochromatic print.

FIG. 21 is a sectional view of a conventional example of image formingapparatus.

FIG. 22 is an enlarged view of a transfer roller.

FIG. 23 illustrates a central dropout.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown an example of an image formingapparatus using an intermediate resistance elastic roller (intermediatetransfer roller) 20, in an image forming apparatus shown in FIG. 21. Theimage forming apparatus of this example is in the form of a color imageforming apparatus (copying machine or a laser beam printer) using anelectrophotographic process.

(1) Designated by a reference numeral 1 is a rotatable drum typeelectrophotographic photosensitive member (photosensitive drum) which isrepetitively usable as a first image bearing member, and it comprises aphotosensitive layer, an electrically conductive base member whichfunctions to support the photosensitive layer which is electricallygrounded. It is rotated in the indicated clockwise direction at apredetermined peripheral speed (process speed).

During rotation of the photosensitive drum 1, it is uniformly charged toa predetermined polarity and potential by a binary charger (coronadischarger) 2, and is exposed to image light 3 by unshown image exposuremeans (a color separation and imaging exposure optical system for colororiginals or a scanning exposure system including a laser scanner foremitting a laser beam modulated in accordance with time series electricdigital pixel signals representative of image information to be printed)for a first color component (magenta component) of the color image to beprinted. Thus, an electrostatic latent image for the color is produced.

Subsequently, the electrostatic latent image is developed with magentatoner M (coloring charged particles) (first color) by a first developingdevice 41 (magenta developing device). At this time, the second tofourth developing devices 42, 43 and 44 (cyan, yellow and blackdeveloping devices) are in an inoperative state, and therefore, they donot act on the photosensitive drum 1, so that the first color magentatoner image is not disturbed by the developing devices 42, 43 and 44.

In this embodiment, the intermediate transfer roller 20 comprises anelectrically conductive core metal 21 in the form of a pipe and anelastic layer 22 having an intermediate resistance formed around thecore metal 21.

The intermediate resistance elastic layer 22 comprises elastic materialsuch as silicone rubber, tetrafluoroethylene rubber, chloroprene rubber,urethane rubber, EPDM (terpolymer of ethylene propylene dien), and metaloxide such as carbon or zinc oxide or the like dispersed therein so thatthe electric resistance (volume resistivity) is intermediate (10⁵ -10¹¹ohm.cm). It is solid or foamed material.

In the apparatus of this embodiment, a transfer material (sheet) 24having an A4 size is longitudinally fed, and the intermediate transferroller 20 has a peripheral length which is slightly larger than thelength of the A4 size transfer material 24 (314 mm, roller outsidediameter is 100 mm). The elastic layer 22 has a thickness of 8 mm and ahardness of 20-40 degrees (Asker C hardness).

The intermediate transfer roller 20 is supported in parallel with thephotosensitive drum 1, and is contacted to a bottom surface of thephotosensitive drum 1. It is rotated at the same peripheral speed as thephotosensitive drum 1 in the indicated counterclockwise direction. Whilethe first color magenta toner image formed on the photosensitive drum 1passes through a transfer nip N where the drum 1 and the intermediateroller 20 are contacted to each other, the image is continuouslytransferred to an outer surface of the roller 20 by an electric fieldformed in the transfer nip N by the transfer bias applied to the roller20.

The surface of the photosensitive drum 1 from which the first colormagenta toner image has been transferred to the intermediate transferroller 20 is cleaned by a cleaning device 14.

Similarly, the above process is repeated as follows:

(2) Charging of the rotating photosensitive drum 1; image exposure 3corresponding to a second component image (cyan component image, forexample); development with cyan toner C by a second developing device 42(cyan developing device); transfer of a second color cyan toner imageonto an intermediate transfer roller 20; and cleaning of thephotosensitive drum 1 surface by a cleaning device 14;

(3) Charging of a rotatable photosensitive drum 1; image exposure of athird color component image (yellow component image, for example);development with yellow toner Y by a third developing device 43 (yellowdeveloping device); transfer of a third yellow toner image onto theintermediate transfer roller 20; and cleaning of the photosensitive drum1 surface by the cleaning device 14; and

(4) Charging of the rotatable photosensitive drum 1; image exposure 3 ofa fourth color component image (black, for example); development withblack toner B by a fourth developing device 44 (black developingdevice); transfer of a fourth color black toner image onto theintermediate transfer roller 20; and cleaning of the photosensitive drum1 surface by the cleaning device 14.

By the sequential execution of the imaging and transfer cycles (1)-(4),the four toner images (magenta, cyan, yellow and black toner images) aresequentially and superposedly transferred onto the intermediate transferroller 20, and therefore, a combined color toner image (mirror image)corresponding to the intended color image is formed.

Designated by a reference numeral 25 is a transfer roller supported inparallel with the intermediate transfer roller 20 so as to be contactedto the bottom surface of the intermediate transfer roller 20, and it isrotated at the same peripheral speed as the intermediate transfer roller20 in the indicated clockwise direction.

The transfer roller 25 comprises a core roller 26 and a thin partinglayer 27 formed on the outer peripheral surface thereof. The partinglayer 27 is of fluorine resin such as PFA or PTFE or the like. It has athickness of 20-100 microns.

The sequential toner image formation process for the first to fourthcolors to the photosensitive drum 1, are the same as in the case of FIG.21. The toner is a non-magnetic one component toner having a volumeaverage particle size of 5-8 microns, and the specific charge thereof is-8--18 μc/gr, and it is chargeable to the negative polarity.

The sequential and superposing transfer of the toner images for thefirst to fourth colors from the photosensitive drum 1 onto the outersurface of the intermediate transfer roller 20 is carried out while thecore metal 21 of the intermediate transfer roller 20 is supplied with atransfer bias voltage of the polarity opposite (+) from that of thetoner from a bias voltage source 61. The applied voltage was in thisembodiment +2 KV-+5 KV.

In the sequential transfer process for the first to fourth color tonerimages from the photosensitive drum 1 to the intermediate transferroller 20, the core metal 26 of the transfer roller 25 is supplied witha bias voltage of the same polarity (-) as the toner from a second biasvoltage source 29. The bias voltage produces a repelling electric fieldfor repelling the toner from the transfer roller 25 to the intermediatetransfer roller 20, so that the transfer of the toner image from theintermediate transfer roller 20 to the transfer roller 25 is prevented.

When the toner image is transferred from the photosensitive drum 1 ontothe intermediate transfer roller 20, the potential of the non-imageportion is different from that of the toner image portion. With thepotential of the core metal 21 being reference potential, the potentialdifference from the non-image portion is larger than the potentialdifference from the toner image portion, and therefore, the transfercurrent flows more into the non-image portion.

This tendency is remarkable when the resistance of the intermediatetransfer roller 20 is low. For example, when the current into thenon-image portion is not less than twice the current into the tonerimage portion, the electric field in the non-image portion influencesthe toner image with the result of the toner scattered around the tonerimage. In other words, a low resistance roller 20 is not suitable for anintermediate transfer member.

On the contrary, in the case of a high resistance roller, the electricfield capable of being formed by the bias voltage source 61 is too smallwith the result of the function of the intermediate transfer rollerdeteriorated. As a result of experiments and investigations by theinventors, in order to prevent the scattering and improper transfer ontothe intermediate transfer member, a volume resistivity of a surfacelayer on the core metal of the intermediate transfer member ispreferably 10⁵ -10¹¹ ohm.cm, preferably 10⁷ -10¹⁰ ohm.cm (intermediateresistance layer). If the elastic layer 22 has the volume resistivity ofthis range, the proper transfer current can be provided by theapplication of +2-+5 KV to the core metal 21.

The first to fourth color toner images superposedly transferred onto theintermediate transfer roller 20 from the photosensitive drum 1, istransferred onto a transfer material 24 fed at predetermined timing intoa nip n between the intermediate transfer roller 20 and the transferroller 25. The transfer material is fed out one-by-one from a sheetfeeding cassette 9 by a sheet feeding roller 10, and is supplied to thetransfer nip n by a pair of registration rollers and along a transferguide 22. When the transfer material reaches the nip n, the bias voltagesource is switched from the second bias voltage source 29 to the firstbias voltage 28, and the core metal 26 is supplied with a transfer biasvoltage of the polarity (+) polarity from that of the toner. An absolutevalue of the transfer bias voltage is larger than the bias voltage ofthe polarity (+) opposite from that of the toner, applied from the biasvoltage source 61 to the core metal 21 of the intermediate transferroller. By the transfer bias voltage, the toner image is transferredfrom the intermediate transfer roller 20 onto the transfer material 24supplied into the nip n between the intermediate roller 20 and thetransfer roller 25. The transfer material 24 now having the toner imagetransferred thereto is introduced into an image fixing device 15 whereinit is heated and pressed by a temperature-controlled fixing roller 16and a pressing roller 17 into a fixed color image. Then, it isdischarged as a color print.

When the transfer material 24 passes through the nip n, the bias voltagesources for the core metal 21 and the core metal 26 are switched to thesecond bias voltage sources 62 and 29 of the same polarity (-) as thetoner.

By the switching of the bias, the toner remaining on the surface of theintermediate transfer roller 20 is returned onto the photosensitive drum1 surface, and it is collected by the cleaning device 14, by which theintermediate transfer roller 20 surface is cleaned.

The toner deposited on the transfer roller 25 surface is returned to theintermediate roller 20, and is further returned onto the photosensitivedrum 1 surface, and therefore, it is also collected by the cleaningdevice 14. Thus, the transfer roller 25 is also cleaned.

As described, the intermediate transfer roller 20 has an intermediateresistance elastic layer 22, and therefore, the remaining toner issufficiently returned to the photosensitive drum 1 surface by the biasvoltage application (cleaning bias) of the same polarity as that of thetoner. Thus, the toner is effectively transferred back to clean itwithout use of a particular cleaning device, thus simplifying thestructure of the apparatus.

The intermediate transfer roller 20 is driven by a gear (not shown)integral with the core metal 21. As contrasted to the case of theintermediate transfer belt, there is no need for using a complicatedmechanism such as a lateral shift preventing mechanism, thus simplifyingthe structure of the apparatus.

EMBODIMENT 2

(FIGS. 2-4)

In the apparatus of the first embodiment (FIG. 1), the image transferfrom the intermediate transfer roller 20 onto the transfer material 24is carried out while the transfer material 24 is passed through the nipn between the intermediate transfer roller 20 and a transfer roller 25press-contacted thereto. In this embodiment, as shown in FIG. 2, thetransfer material 24 is fed into the transfer nip N between thephotosensitive drum 1 and the intermediate transfer roller 20, and thereis no need to use the transfer roller 25.

The sequential and superposing transfer of the toner image of the firstto fourth color toner images from the photosensitive drum 1 onto theintermediate transfer roller 20 is carried out while a transfer bias ofthe opposite (+) polarity from that of the toner from the first biasvoltage source 21 to the core metal 21 of the intermediate transferroller 20.

The transfer of the toner image from the intermediate transfer roller 20to the transfer material 24 is carried out while the core metal 21 issupplied with a bias voltage of the same (-) polarity as that of thetoner after the bias voltage source is switched from the first biassource 61 to the second bias source 62 as shown in FIG. 3.

In the case of a monochromatic print, as shown in FIG. 4, the transfermaterial 24 is supplied at the predetermined timing into a transfer nipN between the photosensitive drum 1 and the intermediate transfer roller20 without transfer of the toner image (monochromatic toner image t_(a)from the photosensitive drum 1 onto the intermediate transfer roller 20.By the application of a transfer bias of the same polarity as that ofthe toner from the first bias voltage source 61 to the core metal 21 ofthe intermediate transfer roller 20, the toner image t_(a) may bedirectly transferred onto the surface of the transfer sheet 24 (t_(b)),by which the printing process is simplified.

Also when there is a transfer material 24 between the photosensitivedrum 1 and the intermediate transfer roller 20, there is an imbalance inthe transfer currents for the toner image portion and the non-imageportion, as has been described in connection with the first embodiment,and by the fact that the elastic layer 22 of the intermediate transferroller 20 has an intermediate resistance, a proper and satisfactorytransfer is accomplished.

In either case of FIGS. 3 and 4, the removal of the toner from theintermediate transfer roller 20 after the toner image transfercompletion to the transfer material 24 is carried out by application ofa bias voltage of the same polarity (-) as the toner from the secondbias voltage source 62 to the core metal 21 of the roller 20. By thebias voltage application, the toner remaining on the roller 20 transfersback to the photosensitive drum 1, and is collected by the cleaningdevice 14 for the photosensitive drum 1, thus effecting the cleaning ofthe roller 20.

EMBODIMENT 3

(FIG. 5)

FIG. 5 is a sectional view of an image forming apparatus according to athird embodiment of the present invention. In this embodiment, theintermediate roller 20 is capable of being supplied with a voltage fromthree voltage sources, namely, first, second and third bias voltagesources. The other structures are the same as in the second embodimentapparatus (FIG. 2). Similarly to the second embodiment, the first andsecond bias voltage sources 61 and 62 are voltage sources for applying atransfer bias voltage of the polarity (+) opposite from that of thetoner to the core metal 21 of the intermediate transfer roller 20 duringthe toner image transfer operation from the photosensitive drum 1 to theintermediate transfer drum 20, and a voltage source for applying atransfer bias voltage of the same polarity (-) as that of the toner tothe core metal 21 of the intermediate transfer roller 20 during thetoner image transfer operation from the intermediate transfer roller 20to the transfer material 24.

The third bias voltage source 63 functions as a cleaning biasapplication voltage source. After the completion of the toner imagetransfer from the intermediate transfer roller 20 to the transfermaterial 24, the bias voltage source for the core metal 21 of theintermediate transfer roller 20 is switched from the second voltagesource 62 to the third bias voltage source 63, so that the intermediatetransfer roller 20 is supplied with a cleaning bias voltage of the samepolarity (-) as the toner to the intermediate transfer roller 20, bywhich the toner remaining on the outer peripheral surface of theintermediate transfer roller 20 is removed therefrom to be transferredback to the photosensitive drum 1 surface at the transfer nip N. Thetoner returned to the photosensitive drum 1 is collected by the cleaningdevice 14, and therefore, the surface of the photosensitive drum 1 isalso cleaned. The third bias voltage source 63 may have a lower outputvoltage than the second bias voltage source 62, and when there is notransfer material in the nip N, the excessive charge application fromthe roller 20 to the drum 1 can be prevented.

EMBODIMENT 4

(FIGS. 6 and 7)

The apparatus of this embodiment is capable of printing on both sides ofthe transfer material 24. In FIG. 6, designated by reference numerals 45and 46 are a negative developing device containing negative polaritytoner and a positive developing device containing positive toner.

A latent image for the image information for a first side is firstformed on the surface of the photosensitive drum 1, and the latent imageis developed by the negative developing device 45 (the positivedeveloping device 46 is in an inoperative state). The negative polaritytoner image is transferred temporarily onto the outer peripheral surfaceof the intermediate transfer roller 20. During the intermediate transferoperation, the core metal 21 of the roller 20 is supplied with apositive polarity bias voltage from the first bias voltage source 61.

Subsequently, a latent image of image information for the second side isformed on the surface of the photosensitive drum 1, the latent image isdeveloped by the positive developing device 46, and the negativedeveloping device 45 is in an inoperative state.

In synchronism with the formation of the toner image, the transfermaterial 24 is fed to the transfer nip N formed between thephotosensitive drum 1 and the intermediate transfer roller 20 from thesheet feeding cassette 9.

Immediately before the transfer material 24 enters the transfer nip N,the bias voltage source for the core metal 21 of the intermediatetransfer roller 20 is switched from the first bias voltage source 61 tothe second bias voltage source 62, so that the core metal 21 is suppliedwith a negative polarity bias voltage. As shown in FIG. 7, by doing so,the negative polarity toner image t_(c) carried on the intermediatetransfer roller 20 surface is transferred onto the first side (bottomside) of the supplied transfer material 24, and simultaneously, thepositive polarity toner image t_(d) is transferred from thephotosensitive drum 1 onto the second side (top side) of the suppliedtransfer material 24.

The transfer material 24 discharged from the transfer nip N has thetoner images on both sides of the transfer material. To prevent rubbingcontact between the toner image and other parts until it is introducedinto the fixing device 15, the conveyance guide 18 provides upward airflow by a fan 13a. The toner images on the opposite sides of thetransfer material 24 are fixed by the fixing device 15.

By utilizing a roller 20 as the intermediate transfer material, bothside copy (duplex) print can be accomplished with a simplified sheetconveying arrangement.

In the case of a simplified mode (one side print), either the negativedeveloping device 45 or positive developing device 46 is used to form atoner image on the photosensitive drum 1, and on the other hand, thetransfer material 24 is supplied to the transfer nip N, while the coremetal 21 of the intermediate transfer roller 20 is supplied with a biasvoltage of the polarity opposite from that of the toner from the firstor second bias voltage source 61 or 62.

In the case of the transfer of different polarity, the intermediateresistance roller 20 (intermediate transfer member) provides astabilized image transfer function, and therefore, satisfactory imagescan be provided. In the foregoing explanation, an intermediate transferroller 20 having a single elastic layer 22. However, multi-layerstructure is usable for the elastic layer 22, or the elastic layer 22may be coated with smooth surface layer.

As has been described in connection with embodiments 1, 2, 3 and 4, byusing the intermediate elastic material roller 20 as the intermediatetransfer member or roller, the disturbance of the transferred image canbe prevented, and in addition, an image forming apparatus using anintermediate transfer member can be accomplished with simple structure.

In the following embodiments 5, 6, 7 and 8, the central dropout of thetoner image can be prevented.

EMBODIMENT 5

(FIGS. 8 and 9)

FIG. 8 shows an image forming apparatus according to this embodiment,which is similar to the apparatus of the first embodiment (FIG. 1).

The intermediate transfer roller 20, in this embodiment, comprises acore metal 21 in the form of a pipe, and a resistance layer 22 on theouter surface of the core metal 21. The resistance layer 22 comprisingaramide resin, polycarbonate or fluorine resin material and fine carbonor metal powder uniformly dispersed therein so that the volumeresistivity thereof is 10⁷ -10¹⁰ ohm.cm.

The peripheral length of the intermediate transfer roller 20 is selectedso as to be slightly larger than the length of the transfer sheet 24. Inthis embodiment, the transfer sheet 24 having the A4 size can belongitudinally fed, and therefore, the outside diameter of the roller 20is 100 mm (peripheral length of 314 mm). The resistance layer 22 has athickness of 100 microns.

The transfer roller 25 comprises a core metal and an elastic layer 27formed on the peripheral surface thereon, the elastic layer 27comprising carbon dispersed EPDM foamed material having a volumeresistivity of approx. 10⁹ ohm.cm. The hardness thereof is not more than40 degrees (Asker C).

The sequential superposing transfer operations for the first to fourthcolors from the photosensitive drum 1 onto the intermediate transferroller 20, are the same as in the case of FIG. 1, and the core metal 21of the intermediate transfer roller 20 is supplied with a transfer biasvoltage of +2-+5 KV of the polarity (+) opposite from that of the tonerfrom the bias voltage source 61.

In this embodiment, during the transfer process, the transfer roller 25is spaced (non-contact) from the intermediate transfer roller 20, thuspreventing disturbance of the toner image on the intermediate transferroller 20.

Designated by a reference numeral 8 is a cleaning device for theintermediate roller 20, and it is kept out of contact with theintermediate transfer roller 20 when the intermediate transfer rollercarries the toner image.

The peripheral speed of the intermediate transfer roller 20 at the nip Nis approx. 0.5% higher than the peripheral speed of the photosensitivedrum 1 at the nip N. By doing so, the toner image is efficientlytransferred from the photosensitive drum 1 onto the intermediatetransfer roller 20, so that the central-image drop-out can be prevented.

For the transfers of the first to fourth color toner images onto thetransfer material 24 from the intermediate transfer roller 20, thetransfer roller 25 is brought into contact with the intermediatetransfer roller 20 prior to the leading edge of the toner image on theintermediate transfer roller 20, and the core metal 26 of the transferroller is supplied with a transfer bias voltage of the polarity (+) thesame as that of the toner, from a bias voltage source 28.

Subsequently, the transfer material 24 enters the nip n between theintermediate transfer roller 20 and the transfer roller 25, so that thetoner image is transferred from the intermediate transfer roller 20 tothe transfer material 24.

The proper voltage of the transfer bias to the core metal of thetransfer roller 25 depends on the transfer voltage applied to the coremetal 21 of the intermediate transfer roller 20.

More particularly, the transfer bias voltage to the core metal 26 of thetransfer roller 25 is preferably 2-4 KV higher in absolute value thanthe transfer bias voltage applied to the core metal 21 of theintermediate transfer roller 20, since then proper image transfer can beaccomplished.

Since the voltage applied to the core metal 21 of the intermediatetransfer roller 20 is +500 V when the toner image is transferred fromthe intermediate roller 20 to the transfer material 24 in thisembodiment, the voltage applied to the core metal 26 of the transferroller 25 is selected to be 3 KV.

In this embodiment, the peripheral speed at the nip of the transferroller 25 is 0.5% higher than that of the peripheral speed of theintermediate transfer roller 20 at the nip, so that the conveying speedof the transfer material 24 is higher than the peripheral speed of theintermediate transfer roller 20. By doing so, the transfer of the tonerimage from the intermediate transfer roller 20 to the transfer material24 is efficiently carried out with prevention of the central dropout.

Therefore, the central dropout can be prevented in the transfer from thephotosensitive drum onto the transfer material 24 through theintermediate transfer member 20.

In this embodiment, the toner image on the photosensitive drum 1 isexpanded approx. by 1.5% on the transfer material 24 in the sheetconveyance direction. However, this can be compensated by adjustingmagnification upon the formation of the latent image on thephotosensitive drum, that is, the latent image is formed with reductionof 1.5%.

Here, the description will be made as to the drop-out preventing effectusing the peripheral speed difference in this embodiment. FIG. 9 shows arelationship between the degree of the central dropout of thetransferred image and the peripheral speed of the intermediate transferroller 20 at the nip when the peripheral speed of the photosensitivedrum 1 at the nip N is 100. As will be understood from this Figure, thedegree of the central dropout is improved by the peripheral speeddifference. It is considered that the shearing force pending to wipe outthe toner image by the peripheral speed difference is effective toimprove the transfer efficiency. The same tendency exists in thetransfer from the intermediate transfer roller 20 and the transfermaterial 24. In this embodiment based on the results, the peripheralspeed of the intermediate transfer roller 20 is made higher by 0.5% thanthat of the photosensitive drum 1, the peripheral speed of the transferroller 25 is made higher by 0.5% than that of the intermediate transferroller 20, and it has turned out that the proper image transfer ispossible.

EMBODIMENT 6

This embodiment is similar to Embodiment 5 (FIG. 8), but the peripheralspeed of the intermediate transfer roller 20 is lower by 1% than that ofthe photosensitive drum, and the peripheral speed of the transfer roller25 is higher by 0.5% by the intermediate transfer roller 20. By doingso, the central dropout can be prevented both in the transfer from thephotosensitive drum to the intermediate transfer roller and in thetransfer from the intermediate transfer roller to the transfer material.In addition, the peripheral speed differences compensate with each otherthe magnification change in the sheet moving direction.

More particularly, the toner image on the photosensitive drum 1 isreduced by 1% on the intermediate transfer roller 20 by the imagetransfer. When the image is transferred onto the transfer material 24,the transfer roller 25 is rotated at a speed higher by 1.5%, thetransfer material 24 is advanced at a speed approx. 1% higher than theperipheral speed of the intermediate transfer roller 20 between theintermediate transfer roller 20 and the transfer roller 25 providing alarger friction force. In combination, the image on the transfermaterial 24 has the same size as that on the photosensitive drum 1.Because of the compensating function, a relatively large peripheralspeed difference can be provided, and therefore, the central dropoutpreventing effect can be enhanced accordingly.

EMBODIMENT 7

(FIG. 10)

FIG. 10 shows an image forming apparatus according to Embodiment 7, inwhich the toner image transfer onto the transfer material 24 is effectedin the transfer nip N formed between the photosensitive drum 1 and theintermediate transfer roller 20. This embodiment is similar to thesecond embodiment (FIG. 2).

As shown in FIGS. 3 and 4, the apparatus is operable in a transfer modefor the four color toner image transfer from the intermediate transferroller 20 to the transfer material 24, and a direct transfer mode for amonochromatic toner image transfer from the photosensitive drum 1 to thetransfer material 24.

The elastic layer 22 of the intermediate transfer roller 20 has a volumeresistivity of 10⁷ -10¹⁰ ohm.cm.

In this embodiment, the intermediate transfer member is in the form ofan elastic roller 20, so that the feeding of the transfer material 24 bythe transfer nip N is possible to realize the size reduction of theapparatus. If the peripheral speed of the intermediate transfer roller20 at the nip N is made lower by 0.5% than that of the peripheral speedof the photosensitive drum 1, the speed difference is provided alsobetween the intermediate transfer roller 20 and the transfer material24, so that the central dropout in the transfer from the photosensitivedrum to the intermediate transfer member and from the intermediatetransfer member to the transfer material. This is effective in the caseof monochromatic transfer, but it is particularly preferable in the caseof the four color transfer operation since the expansion or enlargementof the image can be reduced.

This embodiment can be modified so that the peripheral speed of theintermediate transfer member is changed depending on the operationalmode, that is, depending on whether four color images are transferredonto the intermediate transfer member or an image is transferred to thetransfer material, so as to prevent the central drop-out and to increasethe printing accuracy.

For example, the peripheral speed of the intermediate transfer roller 20is 0.5% lower than that of the photosensitive drum 1 during theintermediate transfer operation and is higher by 1% during the transferto the transfer material. By doing so, for the same reasons as describedin conjunction with Embodiment 6, the image on the transfer material 24has the same size as that on the photosensitive drum 1. In addition, theperipheral speed difference from the sheet feeding speed of the transfermaterial 24 during the transfer operation can be sufficiently provided,and therefore, the central drop-out preventing effect is enhanced.

EMBODIMENT 8

(FIG. 11)

FIG. 11 shows an image forming apparatus according to Embodiment 8. Inthis embodiment, in comparison with the apparatus of the seventh (FIG.10) embodiment, the outside diameter of the intermediate transfer roller20 is small.

More particularly, in this embodiment, the peripheral length of theintermediate transfer roller 20 is shorter than the length of thetransfer material 24 by utilizing to the advantage the expansion andreduction of the image in the transfer from the photosensitive drum 1 tothe intermediate transfer material 20 and in the transfer from theintermediate transfer member 20 to the transfer material 24 providingthe speeds differences.

Furthermore, the peripheral speed of the intermediate transfer roller 20is 50% of that of the transfer drum 1 during a four color intermediatetransfer operation, by which the central drop-out is prevented, andallowing reduction of the size of the image on the intermediate transferroller 20.

When the image is to be transferred onto the transfer material 24, thetransfer material 24 is supplied into the transfer nip N by registrationrollers 11 at the same peripheral speed as the photosensitive drum 1,and the toner image is expanded to the original size. At this time,between the transfer material 24 and the intermediate transfer roller20, there is a speed difference during the intermediate transfer, andtherefore, the central drop-out can be prevented.

Generally speaking, when the intermediate transfer member is in the formof a roller 20, the peripheral length of the roller 20 is preferablylonger than the length of the transfer material 24. According to thisembodiment, it becomes possible to use an intermediate transfer roller20 having a peripheral circumferential length which is one half thelength of the transfer material 24. In other words, the diameter thereofcan be reduced to one half, thus permitting size reduction of theapparatus.

In the case of A4 size sheet feeding, the intermediate transfer roller20 is normally required to have 100 mm diameter. This means difficultyin the separation. According to this embodiment, however, the diameterof the intermediate transfer roller 20 can be reduced to 50 mm, andtherefore, the large curvature thereof permits easy separation of thetransfer material, and therefore, the malfunction in the sheetconveyance such as jam can be prevented.

As described in the foregoing, after the image formed on the first imagebearing member is transferred onto the intermediate transfer member at afirst transfer position, it is further transferred onto a second imagebearing member at a second transfer position. In such an image formingapparatus, according to the above-described embodiments, the surfacemovement speed of the intermediate transfer member at the first transferposition is made different from the surface moving speed of the firstimage bearing member at the first transfer position, so that the centraltransfer failure (drop-out) can be prevented. In addition, by thedifference between the surface movement speed of the intermediatetransfer member and the surface moving speed of the second image bearingmember at the second transfer position, the transferred image is freefrom the central drop-out. In this case, the intermediate transfermember may be in the form of a belt as shown in FIG. 21.

EMBODIMENTS 9-11

Which will be described hereinafter are concerned with improvements inthe apparatus using an intermediate resistance roller as theintermediate transfer member, as in the first-eighth embodiments.

(1) The intermediate elastic layer 22 of the intermediate transfermember 20 has a controlled resistance which has been controlled bydispersing carbon or metal oxide or the like in an elastic material.Such an intermediate resistance elastic layer 22 may have varyingresistance due to the manufacturing tolerance or ambient conditionchange. It is desirable the intermediate transfer operation is carriedout with stability even when the resistance of the intermediate elasticlayer 22 varies.

In the case of a constant voltage control in which a constant voltage issupplied to the intermediate transfer roller 20 during the transferoperation to the intermediate transfer member 20, sufficient currentdoes not flow to the photosensitive drum 1 when the resistance of theintermediate transfer member 20 is high under the low humiditycondition, and therefore, it is not possible to sufficiently transferthe toner image from the photosensitive drum 1 onto the intermediatetransfer member with the result of improper image transfer. On thecontrary, under the high humidity condition, if the resistance,of theintermediate transfer member 20 decreases, too much current flows intothe photosensitive drum 1 with the result of drum memory, and backtransfer of the toner image in which the transferred toner istransferred back to the photosensitive drum.

Because the optimum transfer voltages for the first, second, third andfourth toner images are different, the result is difficulty in thecontrol system.

(2) Under a high humidity condition, the resistance of the intermediatetransfer member 20 decreases, and in addition, the resistance of thetransfer member 24 significantly decreases, and the optimum transfervoltage varies when the image is transferred from the intermediatetransfer member 20 onto the transfer material 24.

For these reasons, a transfer voltage which is proper under the normalhumidity condition is too high under the high humidity condition underwhich the resistance of the transfer material 24 greatly decreases withthe result that the transfer current flows through the thickness of thetransfer material, or that the transfer current does not flowsufficiently in the toner present area of the intermediate transfermember 20, so that the transfer current is concentrated on such areas asnot having the toner. If this occurs, the image may scatter.

These problem do not easily arise when the resistance of theintermediate transfer member 20 is relatively high, because the entireresistance including the photosensitive drum 1, the intermediatetransfer member 20, and the transfer material is not easily influencedby the resistance of the transfer material 24. However, when theresistance of the intermediate transfer member 20 is high, the entireresistance is significantly influenced by the resistance change of thetransfer material 24. Particularly, under the high humidity and hightemperature condition, the resistance of the intermediate transfermember 20 tends to decrease to 1/2-1/3. This is a main cause ofpromoting the above phenomenon.

(3) In an image forming apparatus capable of a automatic duplex imageforming function or a superposing image forming function, the imagetransfer for the first surface (or first image) on the transfer material24 is carried out, and the transferred image is fixed, by which themoisture is removed from the transfer material 24 so that a highresistance state arises.

When the transfer material 24 is supplied to the transfer station forthe purpose of the image transfer on the second surface (second image),the resistance of the transfer material 24 is high. Therefore, upon theimage transfer on the second surface (second image) in good order, atransfer voltage which is higher than the transfer voltage capable ofproviding proper transfer for the first surface (first image), isdesired. The Embodiments 9-11 are related to the solution to theseproblems in the apparatus using the intermediate resistance roller asthe intermediate transfer member.

EMBODIMENT 9

(FIG. 12)

FIG. 12 shows an image forming apparatus according to Embodiment 9 ofthis invention. The structure thereof is similar to that of the secondembodiment (FIG. 2).

The intermediate transfer roller 20 as the intermediate transfer membercomprises a core metal 21 in the form of a pipe and an elastic layer 22formed thereon, the elastic layer 22 comprises an elastic material ofEPDM or the like and fine carbon or metal powder uniformly dispersedtherein so that the volume resistivity thereof is 10⁵ -10¹¹ ohm.cm. Thecircumferential length of the intermediate transfer roller 20 isslightly larger than the length of the transfer material 24. In thisembodiment, the transfer material 24 having an A4 size is longitudinallyfed, and therefore, the intermediate transfer roller 20 has an outerdiameter of 100 mm (circumferential length of 314 mm), and the thicknessof the elastic layer 22 is 8 mm. The hardness thereof is 30-50 degrees(Asker C).

More particularly, the process speed is 90 mm/sec, and the diameter ofthe photosensitive drum 1 is 30 mm.

Designated by a reference numeral 50 is a bias source for anintermediate transfer roller 20. The bias voltage source 50 may be inthe form of a constant current source or a constant voltage source. Inaddition, the polarity is changeable.

Designated by a reference numeral 51 is a bias voltage detecting meansfor the intermediate transfer roller 20; 52 is a CPU; 53 is I/O port;54, memory; and 56, ground.

The control operation of the apparatus in this embodiment will bedescribed.

(1) The sequential control operation is carried out using the CPU 52,and to the core metal 21 of the intermediate transfer roller 20, avoltage V_(t0) is applied to provide a constant electric current I=4 μA,and the intermediate transfer roller 20 is constant-current-controlled,during which the toner image is transferred from the photosensitive drum1 to the intermediate transfer roller 20.

With the advancement of the transfer from the first color to the fourthcolor, the amount of the toner on the intermediate transfer roller 20increases, and therefore, the voltage V_(t0) increases. Even within thefirst color image, the voltage V_(t0) changes depending on the amount ofthe toner. At this time, the voltage V_(t0) has a polarity which isopposite from that of the toner.

(2) Accordingly, the entire color image most suitably reflects theresistance at the time when the fourth toner image is being transferred.During the fourth color transfer, the transfer voltage V_(t0) ismeasured during a constant current bias control operation, and theaverage of the transfer voltage V_(t0) ! during the fourth colortransfer is used as a voltage V_(T) for the control operation.

(3) A proper transfer voltage V corresponding to the measured voltageV_(T) in accordance with a transfer voltage calculating equation (A)with a being a predetermined al and b being a predetermined value b1, asfollows:

    |V|=a|V.sub.T |+b      (A)

When the toner image is transferred from the intermediate transferroller 20 onto the transfer material 24, the intermediate transferroller 20 is constant-voltage-controlled at the calculated transfervoltage V, and the toner transfer onto the transfer material 24 iscarried out with the voltage. At this time, the transfer voltage V hasthe same polarity as the toner.

More particularly, when the constants a and b are set to

a1=1.0

b1=1.2

proper image transfer operation was performed.

The constant b in the equation (A) is added in consideration of avoltage drop due to the transfer material resistance in the transferoperation (sheet passage).

EMBODIMENT 10

The image forming apparatus itself of this embodiment is the same as theninth embodiment. However the controlling operation is different asfollows.

(1) The sequential control operation is carried out under the control ofCPU 52, and to the core metal 21 of the intermediate transfer roller 20,a voltage V_(t0) capable of supplying a constant current of I=4 μA isapplied. The toner image is transferred from the photosensitive drum 1onto the intermediate transfer roller 20 while constant-currentcontrolling the current to the intermediate transfer roller 20.

With the advancement of the transfer process from the first color to thefourth color, the amount of the toner on the intermediate transferroller 20 increases, and the voltage V_(t0) increases. Even within onecolor image, the voltage V_(t0) changes with amount of the toner. Atthis time, the voltage V_(t0) has a polarity which is opposite from thatof the toner.

(2) Therefore, the resistance of the entire color image is most suitablyreflected when the fourth color toner image is being transferred. Thetransfer voltage V_(t0) during the constant current bias control at thistime is measured, and the average V_(t0) ! is used as the controllingvoltage V_(T).

(3) The measured voltage V_(T) is compared with a predeterminedreference voltage (discriminating voltage) V1.

When |V_(T) |≧V1 as a result of comparison, a usual control is carriedout. That is, similarly to Embodiment 9, the proper transfer voltage Vcorresponding to the measured voltage V_(T) is calculated in accordancewith the transfer voltage calculating equation (A) with the constants aand b being predetermined values a1 and b1, respectively.

When the toner image is transferred from the intermediate transferroller 20 onto the transfer material 24, the intermediate transferroller 20 is constant-voltage-controlled with the calculated transfervoltage V so as to transfer the toner image onto the transfer material24. The transfer voltage V at this time has the same polarity as thetoner.

(5) However, if |V_(T) |<V1 in (4), it is determined that the resistanceof the intermediate transfer member 20 decreases under a high humiditycondition, and simultaneously, the resistance of the transfer material24 deceases. Therefore the transfer current may more easily penetratethrough the thickness of the transfer material during the transferoperation, or the toner scattering may occur more easily due to the toolarge transfer current. Accordingly, the constants a and b are changedto a2 and b2 so as to provide a smaller voltage V by the transfervoltage calculating equation (A). Then, the proper transfer voltage Vcorresponding to the measured voltage V_(T) is calculated in accordancewith the transfer voltage calculating equation (A) with the constants aand b being a2 and b2, respectively.

When the toner image is transferred from the intermediate transferroller 20 onto the transfer material 24, the intermediate transferroller 20 is constant-voltage-controlled at the calculated transfervoltage V, and the toner image transfer onto the transfer material 24 isexecuted. Here, the transfer voltage V has the same polarity at thetoner.

More particularly, the reference voltage V1 is 1 KV, and the constants aand b in the equation (A) are as follows:

a1=1.0

b1=1.2

a2=2.2

b2=0

By doing so, when the absolute value of the measured voltage V_(T) ishigher than 1 KV (=V1) the calculation of the proper transfer voltage V,the constant voltage control for the intermediate roller with thecalculated voltage V during the image transfer from the intermediatetransfer member 20 to the transfer material 24, as stated in (4) above.

When the measured voltage V_(T) is not more than 1 KV, the calculationof the proper transfer voltage V for providing lower transfer voltage,and the constant voltage control operation for the intermediate transferroller 20 with the calculated voltage V during the transfer operationare carried out.

When the measured voltage V_(T) is 1 KV, a1|V_(t) |+b1=a2|V_(T) |=b2.

Therefore, the continuity of the control voltage is maintained, but thecontinuity is not inevitable. In addition, the voltage which becomes adiscrimination reference for the constants a and b, is not limited toonly one voltage V1, but a plurality of such voltages are usable.

In the transfer voltage calculating equation (A), the constant b isadded in consideration of a voltage drop due to the resistance of thetransfer material during the transfer operation (sheet passage period).

Therefore, when the transfer material resistance decreases due tomoisture absorption of the transfer material 24 under the hightemperature and high humidity condition, the proper transfer voltage Vcan be provided by decreasing the constant b.

EMBODIMENT 11

(FIG. 13)

FIG. 13 shows an image forming apparatus according to Embodiment 11 ofthis invention. The apparatus of this embodiment is similar to theapparatus of the ninth embodiment (FIG. 12).

After the toner image is transferred from the photosensitive drum 1 ontothe intermediate transfer roller 20, the transfer material 24 issupplied into the transfer nip N between the photosensitive drum 1 andthe intermediate transfer roller 20 to receive the toner image on thefirst surface. It is then subjected to the image fixing operation in thefixing device 15. Thereafter, the transfer material 24 is fed back by areversing device 81 of a refeeding device 80 mounted at the bottom ofthe image forming apparatus, along a passage indicated by an arrow c.Thus, the transfer material 24 is introduced to the transfer guide 12along the passage d. It is then fed into the transfer nip N between thephotosensitive drum 1 and the intermediate transfer roller 20, and theimage transfer operation is carried out on the second surface of thetransfer roller 24. When the operation of the reversing device 81 isdeactivated the transfer material 24 is not reversed, and thesuperposing transfer is carried out.

In such an apparatus, as described hereinbefore, the transfer material24 already having the first side image transferred thereto and beingrefed into the transfer nip N for the transfer on the second surface,has been dried by the heat in the fixing device during the first sideimage fixing, so that the resistance of the transfer material 24 ishigher than during the first side image transfer.

Under the high humidity condition, the resistance of the transfermaterial 24 during the first side image transfer is low. In the firstside image transfer operation, as described in conjunction withEmbodiment 10, the transfer voltage is preferably lowered sinceotherwise improper image transfer occurs due to the too high transfervoltage. In other words, there is no condition which satisfies both thefirst side image transfer and the second side image transfer.

In such a case, the constants a and b in the transfer voltagecalculating equation (A) are determined in consideration of thedifference between the first side transfer and the second side transferfor the transfer material 24.

The second side image formation is carried out by the refeedingmechanism 80 after formation of the image on the first surface throughthe processes (1)-(5) in Embodiment 10.

(6) The proper transfer voltage V corresponding to the measured voltageV_(T) is calculated in accordance with the transfer voltage calculatingequation (A) with the constants a and b being predetermined values a3and b3, for the second side image formation mode.

When the toner image is transferred from the intermediate transferroller 20 onto the transfer material 24, the intermediate transferroller 20 is constant-voltage-controlled with the calculated transfervoltage V, and the second image is transferred onto the second surfaceof the transfer material 24. At this time, the transfer voltage V hasthe same polarity as the toner.

With the control explained in (6), the toner can be properly transferredboth in the duplex transfer and the superposing transfer.

More particularly, the control operation has been carried out under thefollowing conditions:

(a) Reference voltage V1 for the comparison is 1.0 KV

(b) The constants a and b of the transfer voltage calculating equation(A) in the first side image forming mode when |V_(T) |≧V1:

a1=1.0

b1=1.0

(c) The constants a and b in the transfer voltage calculating equation(A) in the first side image formation mode when |V_(T) |<V1:

a2=2.2

b2=0

(d) The constants a and b of the transfer voltage calculating equation(A) for the second side image formation mode in the both side imageformation (duplex).

As a result, even if the resistance of the transfer material 24 is lowunder the high humidity condition, the constants a=a2 and b=b2 for theimage transfer onto the first side of the transfer material 24, andsimilarly to the Embodiment 10, the scattering or the currentpenetration due to the too high transfer voltage can be prevented.

For the image transfer onto the second side of the transfer materialhaving a high resistance because of the drying effect during the fixingoperation for the first side transferred image, the constants a=a3, b=b3against the improper transfer due to the insufficient transfer currentdue to the high resistance of the transfer material 24, the transfervoltage without the improper transfer can be provided.

The constant b in the equation (A) is added in consideration of thevoltage drop due to the transfer material resistance in the transfer(sheet passage). Therefore, when the transfer material resistance isincreased by the drying effect of the fixing device 15, the transfervoltage V can be optimized by increasing the constant b.

The above-described Embodiments 9, 10 and 11 are not limited to theimprovements in the transfer performance under the above-described highhumidity condition, but they are usable for optimizing the transferperformance under the normal humidity or low humidity conditions.

According to an image forming apparatus according to Embodiments 9, 10or 11, the image formed on the first image bearing member is transferredonto an intermediate transfer member comprising an intermediateresistance roller, and is further transferred onto a second imagebearing member. Even when the resistance of the intermediate transfermember and/or the resistance of the transfer material significantlychanges in accordance with ambient condition change, an optimum transferproperty can be provided by the application of a constant current biasvoltage to the intermediate transfer member when the image istransferred onto the intermediate transfer member on the first imagebearing member. When the image is transferred from the intermediatetransfer member to the transfer material, the proper transfer propertycan be provided by application of an optimum voltage to the intermediatetransfer member. Furthermore, during the both side and superimposingimage transfer processes there is no difference between the firstsurface (first image transfer) and the second surface (second imageformation), so that the stabilized transfer property can always beprovided.

In Embodiments 12, 13 and 14 which will be described hereinafter, theseparation of the transfer material from the transfer nip is made easierduring the discharging, and the image disturbance during the transfer isprevented.

EMBODIMENT 12

(FIGS. 18-20)

FIG. 18 shows an image forming apparatus according to Embodiment 12.

In the embodiment of this apparatus, the transfer roller 23' functionsalso as an intermediate transfer member. Similarly to Embodiment 2 (FIG.4), the image is transferred from the transfer roller 23' to thetransfer material in the case of multi-color print; and in the case ofmonochromatic print; the image is directly transferred from thephotosensitive drum 1 onto the transfer material.

The transfer roller 23' comprises a core metal 23a, an electricallyconductive elastic layer 23b having a volume resistivity of 10³ -10⁶ohm.cm, and an intermediate resistance thin layer 23c (volumeresistivity of 10⁷ -10¹¹ ohm.cm), so that an intermediate resistanceelastic roller is constituted as a whole. A radius r'_(T) satisfiesL<2πr'_(T) where length of the transfer material 23 is L.

The photosensitive drum 1 has a radius r'_(D), and the curvature thereofis

1/r'_(T) <1/r'_(D)

Similarly to the first embodiment (FIG. 1), the sequential toner imageformations for the first to fourth colors on the photosensitive drum 1,and a sequential superposing transfer on the outer peripheral surface ofthe transfer roller 23' functioning as an intermediate transfer memberof the toner image of the first to fourth colors, are carried out sothat a combined color toner image is formed on the outer surface of thetransfer roller 23'. Similarly to the above-described second embodiment(FIG. 2), the image transfer of the toner image transferred onto theouter peripheral surface of the transfer roller 23' onto the transfermaterial 24 is effected as follows. The transfer material 24 is suppliedat the predetermined timing into the transfer nip N between thephotosensitive drum 1 and the transfer roller 23' from a sheet feedingcassette, and a bias voltage of the same polarity (-) as the toner isapplied to the core metal 23a of the transfer roller 23' from the secondbias voltage source 62. FIG. 1 shows the transfer process.

The transfer material 24 discharged from the transfer nip N is attractedto the transfer roller 23' rather than to the photosensitive drum 1, andsince the transfer material 24 tends to be convex up in the transfer nipN (when the transfer roller 23' is at the upper position), the leadingedge of the transfer material 24 is separated from the transfer roller23' by the rigidity, thus facilitating the separation action.Particularly when the diameter of the photosensitive drum exceeds 60 mm,improper separation tends to occur more frequently, and therefore, it ispreferable to satisfy 1/r'_(T) <1/r'_(D) in this case.

The transfer roller 23' may transfer the toner image t_(a) from thephotosensitive drum 1 directly onto the transfer material 24 (t_(b))bythe transfer roller 23' during a monochromatic mode operation. For thesame reason as described above, the separation of the transfer material24 from the transfer nip N is easy.

EXPERIMENT 1

The photosensitive drum 1 is comprised of aluminum core metal and anorganic photoconductive (OPC) with radius of 35 mm. It is driven at aperipheral speed of 100 mm/sec, and a latent image is formed, andsequentially color toner images are formed.

The black, magenta, cyan and yellow toners were non-magnetic and had avolume average particle size of 5-8 microns (one component toner). Theyare negatively chargeable toners having a specific charge amount of-8--18 μc/gr, and the toner image was produced through a reversedevelopment.

The transfer roller 23' comprised an aluminum core metal 23a having aradius of 40 mm and a foamed EPDM layer 23b in which carbon is dispersedto provide a volume resistivity of 10³ ohm.cm, and a PVdF layer 23ccontaining tin oxide and having a volume resistivity of 10⁹ ohm.cm. Itwas an intermediate resistance roller having a radius of 50 mm. Thehardness of the transfer roller 23' is 35 degrees (Asker C) and waspress-contacted to the photosensitive drum 1 with a total pressure of1000 g including the weight of the transfer roller 23' itself to formthe transfer nip N. The transfer material 24 was plain paper sheet of A4size (L=297 mm) having a basis weight of 90 g/m².

When the toners of each color are transferred onto the intermediateroller, the core metal of the photosensitive drum 1 is grounded, and+1000 V is. added to the core metal 23a of the transfer roller 23'. Whenthe color toner image is transferred from the transfer roller 23' ontothe transfer material 24, -3000 V is applied to the core metal 23a ofthe transfer roller 23'.

In this manner, upon the separation of the transfer material 24 from thetransfer nip N, good transfer image without image disturbance could beprovided without deflection of the transfer material 24 toward thephotosensitive drum 1.

As shown in FIG. 14, the transfer roller 23' may comprise a core metaland one intermediate resistance elastic layer 23b. The curvature1/r'_(T) of the transfer roller 23' is smaller than the curvature1/r'_(D) of the photosensitive drum (1/r'_(T) <1/r'_(D)). When thetransfer material 24 is discharged from the transfer nip N, the transfermaterial 24 receives mirror force toward the photosensitive drum 1 bythe surface charge, and also attraction mirror force toward the transferroller 23' due to the backside charge. As a result of the combination ofthese forces with the rigidity of the transfer material 24 itselftending to be away from either of the surfaces, results in separationwith deviation toward the transfer roller 23' side having the smallercurvature.

In other words, the transfer material 24 moves along a line inclinedfrom the transfer nip N toward the transfer roller 23' from a tangentline at the transfer nip N, so that the transfer material 24 does notrub the bottom of the cleaning device 14 (FIG. 18), or is not jammed.

The transfer material is attracted by the transfer roller 23' whiledischarged from the transfer nip N. However, in the transfer nip N, theelastic material transfer roller 23' is pushed to the rigidphotosensitive drum 1, so that concave up nip is formed (when thephotosensitive drum is at the upper position). When the leading edge ofthe transfer material 24 is discharged from the transfer nip N, such aforce is applied that the leading edge is away from the surface of thetransfer roller 23' by the rigidity of the transfer material 24 in thetransfer nip N, and therefore, the transfer material 24 is preventedfrom being wrapped around the transfer roller 23'.

EMBODIMENT 13

(FIGS. 15 and 16)

FIGS. 15 and 16 show the results of control of the transfer bias for thetransfer onto the transfer material 24 and control sequence, in theapparatus of this embodiment.

Similarly to FIG. 14 embodiment, the transfer roller 23' has a smallercurvature 1/r'_(T) which is smaller than the curvature 1/r'_(D) of thephotosensitive drum 1. In this embodiment, the absolute value of thetransfer bias voltage at the leading or trailing edge of the transfermaterial 24 is reduced, by which the mirror force toward thephotosensitive drum 1 and the transfer roller 23' is reduced at the timeof the discharge of the transfer material 24 from the transfer nip N, bywhich the separation of the transfer material 24 is made smoother.

As shown in FIG. 16, when the print signal is transmitted from a hostcomputer, the photosensitive drum 1 starts rotation, and simultaneouslythe charging of the primary charging roller 2 (FIG. 22) starts.

At the point of time τ₀ when the surface potential of the photosensitivedrum 1 is stabilized, scanning exposure by the laser starts. In the timeperiod δτ₁ corresponding to the length of the transfer material 24, theexposure is not effected during the leading end portion time δτ₂ and thetrailing end corresponding period δτ₃, and therefore, the electrostaticlatent image is not formed on the portions of the drum 1 correspondingto the leading and trailing end portions of the transfer material 24,whereby blanks are formed. The developing bias is applied to thedeveloping device 4, so that developing operation is carried out.However, the developing bias is rendered on from a point of time (τ₀+τ₄) in the region where the image region of the photosensitive drum 1is faced to the developing device 4, and it is kept on for the timeperiod δτ₁, corresponding to the image area, and thereafter, it isdeactivated.

The registration roller 11 starts to rotate τ₅, and the transfermaterial 24 20 reaches the transfer nip N in synchronism with theleading edge of the image region along the transfer guide 12. At thepoint of time τ₆ when the leading edge of the transfer material 24reaches the transfer nip N, the bias voltage starts to be applied to thetransfer roller 23'.

The transfer bias voltage is applied for the time period δτ₁corresponding to the length of the transfer material 24. However, in thetime periods δτ₇ and δτ₈ corresponding to the leading and trailing edgeportions, the voltage applied is weakened. In other words, |V'_(Tr)|<|V_(Tr) | where V_(Tr) is the transfer voltage in the image region andthe V'_(Tr) is the transfer voltage for the leading and trailing edgeportions.

The voltage V'_(Tr) is selected within a range such that the leading andtrailing edges of the transfer material 24 are properly separated inaccordance with the material and shape (curvature 1/r_(T)), it may be 0V.

The transfer voltages V'_(Tr) may be different between the leading edgeand the trailing edge. The leading and trailing edges of the transfermaterial 24 have blanks corresponding to δτ₂ and δτ₄, and therefore,even if the transfer bias is weakened, the improper image transfer doesnot occur, so that good separation performance is accomplished. FIG. 15illustrates the transfer bias change relative to the length of thetransfer material 24 in the conveyance direction.

EMBODIMENT 14

(FIG. 17)

As shown in FIG. 17, the radius r'_(T) of the transfer roller 23' islarge as compared with the radius r'_(D) of the photosensitive drum 1,and the curvatures satisfy 1/r'_(T) <1/r'_(D).

Therefore, the transfer material 24 is attracted more strongly to thetransfer roller 23' than to the photosensitive drum 1.

In this embodiment, an insulative region 18 is provided in at least apart of the surface of the transfer roller 23'. The insulative region 18can be provided by an insulative film of PTFE, PFA, PET or PVdF having agood parting property bonded to the flush with the surface of theelastic layer 23b over the entire length of the transfer roller 23' onthe conductive elastic layer 23b surface. When the length of thetransfer material 24 in the conveyance direction is L, the radius r'_(T)of the transfer roller 23' satisfies L≦2πr'_(T), and when a length onthe circumference of the region 18 is m, 2πr'_(T) -m/L is satisfied. Bydoing so, the leading edge of the transfer material 24 is timed with theregion 18 by the sequential control.

In the region 18, the transfer charge application to the transfermaterial 24 is weakened by the function of the insulative material, andtherefore, the mirror force becomes small, and the leading end portionof the transfer material 24 is not attracted to the transfer roller 23',and therefore, the leading end portion is smoothly separated from thetransfer nip N by the rigidity of the transfer material 24.

When the radius r'_(T) of the transfer roller 23' satisfies the aboverequirement, the trailing edge of the transfer material 24 is timed withthe region 18, and therefore, the trailing end portion is also properlyseparated.

As described in the foregoing, by making the curvature 1/r'_(T) of thetransfer roller 23' having the conductive elastic layer is smaller thanthe curvature 1/r'_(D) of the photosensitive drum (1/r'_(T) <1/r'_(D)),by which the transfer material 24 can be easily separated from thetransfer nip N.

In the Embodiments 12-14, the description has been made in connectionwith a combination of a photosensitive drum I and a transfer roller 23'.However, what is important is the curvatures at the transfer nip N, andtherefore, the photosensitive member functioning as the first imagebearing member is not necessarily in the form of a drum. For example,the present invention is applicable to the belt photosensitive memberwith the use of a rigid backup roller to provide a predeterminedcurvature.

Because the intermediate transfer material has the intermediateresistance, the transfer bias voltage to the intermediate transfermaterial can be properly selected so as to avoid the strong voltage atand adjacent to the transfer nip with the first image bearing member,and to avoid the electric current leakage even if the intermediatetransfer member has fine defects such as pin holes or the like. Then,image disturbance, such as scattering or the like can be prevented, andtherefore, the intermediate transfer can be carried out.

As shown in FIG. 22, when the curvature 1/r_(T) of the transfer roller23 at the transfer nip N is larger than the curvature 1/r_(D) of thephotosensitive drum, the transfer material moves along a line inclinedtoward the photosensitive drum away from a tangent line of the transfernip N. Therefore, the transfer material does not come contact with thecleaning device, and therefore, the contamination or image disturbanceor jamming can be prevented.

While the invention has been described with reference to the structuresdisclosed herein, it is not confined to the details set forth and thisapplication is intended to cover such modifications or changes as maycome within the purposes of the improvements or the scope of thefollowing claims.

What is claimed is:
 1. An image forming apparatus comprising:a rotatable first image bearing member for bearing a toner image; a rotatable intermediate transfer member, wherein a toner image on said first image bearing member is transferred onto said intermediate transfer member at a transfer position, and a toner image on said intermediate transfer member is transferred onto a second image bearing member; and electric field forming means for forming an electric field for transferring residual toner remaining on said intermediate transfer member after the toner image is transferred from said intermediate transfer member onto the second image bearing member, back to said first image bearing member from said intermediate transfer member at said transfer position; wherein a rotational speed of said first image bearing member at said transfer position is different from that of said intermediate transfer member during the back-transfer operation.
 2. An apparatus according to claim 1, wherein the rotational speed of said first image bearing member is lower than that of said intermediate transfer member.
 3. An apparatus according to claim 1, wherein said intermediate transfer member is provided with an electroconductive layer to which a voltage having the same polarity as the toner image by said electric field forming means during the back-transfer operation by said electric field forming means, and provided with a surface layer thereon.
 4. An apparatus according to claim 3, wherein a voltage having a polarity opposite from that of the toner image is applied to said electroconductive layer when the toner image is transferred from said first image bearing member onto said intermediate transfer member.
 5. An apparatus according to claim 3, wherein said surface layer has a volume resistivity of 10⁵ to 10¹¹ Ohm.cm.
 6. An apparatus according to claim 1, further comprising cleaning means for removing toner from said first image bearing member.
 7. An apparatus according to claim 1, wherein said intermediate transfer member is in the form of a roller.
 8. An apparatus according to claim 1, wherein said first image bearing member is capable of carrying a toner image of different colors, which are superimposedly transferred therefrom onto said intermediate transfer member, and the toner images are transferred onto said second image bearing member from said intermediate transfer member. 